vhost: Clean up how VhostOpts method vhost_backend_init() fails

vhost: Clean up how VhostOpts method vhost_backend_init() fails

vhost_user_backend_init() can fail without setting an error.  Unclean.
Its caller vhost_dev_init() compensates by substituting a generic
error then.  Goes back to commit 28770ff935 "vhost: Distinguish errors
in vhost_backend_init()".

Clean up by moving the generic error from vhost_dev_init() to all the
failure paths that neglect to set an error.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-14-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>